Advancement in semiconductor process technologies allows usage of the quadrature bandpass-sampling delta-sigma analog-to-digital demodulator (QBS-ADD) at RF frequencies, which is a new promising low-cost and reliable technique to digitize RF signals directly at the antenna. The technology is based on the bandpass sampling technique combined with delta-sigma modulation to demodulate and digitize an RF signal to baseband in a form of high-sampling-rate bit streams. The down-converted in-phase and quadrature output signals are clocked at a high sampling rate, equal to the carrier frequency that modulates the RF signal. The in-phase and quadrature outputs are bi-level digital signals, having a value of 1 or −1.
Because the demodulated in-phase and quadrature digital signals are sampled at a very high clock rate, generally having a period of sub-nanoseconds, it is possible to exploit the multipath diversity as the granularity of the sampling period can resolve different RF signal paths due to reflections at objects and buildings reaching the quadrature bandpass-sampling receiver. Rake receiving technique can be used in conjunction with the in-phase and quadrature digital output signals, sampled at a high-sampling rate, to provide diversity gain. Furthermore, the 1-bit resolution of the in-phase and quadrature digital outputs allows simple arithmetic operations and avoids usage of digital multipliers.